Increase of degree of spermidine stimulation of polypeptide synthesis in the presence of phosphate

The addition of phosphate caused an increase in the degree of spermidine stimulation of polypeptide synthesis in an Escherichia coli and a wheat germ cell-free system. Optimal stimulation of polypeptide synthesis was observed at 20 mm phosphate for both systems, but concentrations of phosphate up to 40 mm had no additional effect. The increase of degree of spermidine stimulation in the presence of phosphate in an E. coli cell-free system occurred at the level of aminoacyl-tRNA binding to ribosomes and not at the level of peptide bond formation, translocation, or aminoacyl-tRNA formation. From the results of studies on RNase A sensitivity of ribosomal subunits and the effect of antibiotics known to act on the 30 S ribosomal subunits, it is suggested that the nature of the 30 S ribosomal subunits is changed by phosphate so that the degree of spermidine stimulation of polypeptide synthesis is increased.